1. Field of the Invention
This invention relates to a method and apparatus for the generation and processing of trains of pulses, for example to be utilized as interrogating waveforms in object detection systems such as multi-user active sensor systems and particularly, but not exclusively, in automotive radar systems designed to perform functions of obstacle-detection and/or collision-avoidance.
2. Description of the Prior Art
One important type of obstacle-detection/collision-avoidance automotive systems employs short pulses of electromagnetic energy to interrogate the detection zone of interest. A decision regarding the presence or absence of an obstacle at a predetermined range is then made by suitably processing energy backscattered by various objects in the field of view of the system.
FIG. 1 is a block diagram of a conventional obstacle-detection system utilizing short pulses of electromagnetic energy. The system comprises a pulse generator PGR that produces repetitively pulses with duration TP and repetition period TREP. The pulse duration TP is so selected as to provide a required range resolution c TP/2, where c is the speed of light; the unambiguous range of the system is equal to c TREP/2.
The system of FIG. 1 also includes an oscillator OSC that generates a sinusoidal signal with required carrier frequency, a pulse modulator PMD that modulates the carrier signal in an on-off fashion, a power amplifier PAM that amplifies the pulsed carrier signal to a required level, a transmit element TEL that radiates pulses of electromagnetic energy towards an obstacle OBS, a suitable receive sensor REL that receives electromagnetic pulses reflected back by the obstacle OBS, an input amplifier IAM that amplifies the signal provided by the receive sensor REL, a signal conditioning unit SCU that employs suitable signal processing to shape the received pulses, and a pulse-coincidence correlator PCC that processes jointly delayed reference pulses s(t) supplied by the generator PGR and reconstructed pulses r(t) supplied by the signal conditioning unit SCU to obtain an output value dependent on the extent to which received pulses r(t) coincide with reference pulses s(t) and thus provide a decision DEC regarding the presence or absence of an obstacle at a predetermined range corresponding to the delay applied to the reference pulses s(t). The operation can be repeated for other delay values.
It is known that object detectability can be improved significantly when the average power of an interrogating waveform is increased. In the case of electromagnetic pulses with predetermined duration and limited peak power, this can only be achieved if a basic periodic pulse train is replaced by an interrogation signal in the form of a specially constructed composite pulse train comprising unequally spaced (staggered) pulses. While the basic pulse train employs only one pulse per period, the number of pulses occurring within a single period of the composite pulse train can be much greater than one. However, in order to preserve the same unambiguous range of the system, the autocorrelation function of the composite pulse train will have to exhibit low values between its periodically occurring peaks.
As well known to those skilled in the art, various techniques have been developed for constructing pulse sequences with good autocorrelation properties (see for example, P. Fan and M. Darnell, Sequence Design for Communications Applications. Wiley, 1996).
In a multi-user environment, active sensor systems may transmit their own interrogating pulse trains simultaneously and asynchronously so that not only must each system recognize and detect responses to its own transmitted pulses, but it must be able to do so in the presence of all other transmitted pulse trains. For example, in automotive applications, many similar obstacle-detection systems should be capable of operating in the same region, and also be capable of sharing the same frequency band. To avoid mutual interference, each sensor system should use a distinct pulse train, preferably uncorrelated with the pulse trains employed by all other systems. However, because it is not possible to predict which of the many similar systems will be operating in a particular region, it is not practical to assign a distinct pulse train to each of them.
The problem of constructing a large set of composite pulse trains from few underlying ‘template’ pulse trains can be solved, at least partly, by exploiting in a judicious way some random or pseudorandom mechanism in the process of pulse train generation. For example, EP-A1-1330031 and WO-A1-2005/006014 disclose methods which exploit various random mechanisms to generate large sets of composite pulse trains well suited to multi-user applications. The contents of these patent applications are incorporated herein by reference. (Hereinafter, the term “random” is intended to include, where context permits and without limitation, not only purely random, non-deterministically generated signals, but also pseudo-random and/or deterministic signals such as the output of a shift register arrangement provided with a feedback circuit as used in the prior art to generate pseudo-random binary signals, and chaotic signals.)
According to the method disclosed in the above-mentioned patent applications, a composite pulse train consists of a sequence of primary pulse packets each of which is drawn at random from a predetermined set of suitably constructed primary pulse packets with prescribed properties. Consequently, although each user may have at its disposal the same set of primary pulse packets, a composite pulse train transmitted by each user is unique, being assembled in a random manner.
Short electromagnetic pulses transmitted by some obstacle-detection/collision-avoidance automotive systems may have the same duration and amplitude, yet the pulses can still be discriminated on the basis of their carrier frequency, phase, polarisation, or in the case of carrier-less ultrawideband (UWB) systems, their polarity.
One class of obstacle-detection collision-avoidance systems employs pulse trains obtained by suitable encoding of pseudorandom binary sequences, well known to those skilled in the art (see for example, R. C. Dixon, Spread Spectrum Systems with Commercial Applications. Wiley, 1994). One such prior-art construction, intended for UWB systems, is depicted in FIG. 2; as seen, each binary symbol of an underlying pseudorandom binary sequence is represented by a pulse ‘doublet’ or its reversed-polarity replica. Another technique, disclosed in U.S. Pat. No. 6,693,582, makes use of both amplitude shift keying and phase shift keying to provide an improved utilization of pseudorandom binary sequences in automotive radar systems. The contents of U.S. Pat. No. 6,693,582 are incorporated herein by reference.
It would be desirable to provide an improved method for the generation and processing of trains of pulses, particularly pulses suitable to be utilized by active sensor systems designed to perform functions of obstacle-detection and/or collision-avoidance in a multi-user environment.